1. Field of the Invention
This invention relates in general to the field of nuclear reactor control systems and in particular to systems for controlling the operation of control rods which relate to reactor operation and control.
2. Description of the Prior Art
The general details of construction and operation of commercial pressurized water nuclear reactors are well known. Indeed, heat generated by such plants has successfully been converted to electrical power for not an insignificant number of years. Similarly, the manner and means to control the nuclear reaction and the power output from such nuclear reactors, namely: control rods, are also well known. A brief description of the construction and function of control rods is, however, beneficial for a full understanding of the invention.
Control rods each typically comprise a plurality of elongated rods which are parallel arranged around and attached to a central shaft. The control rods are insertable into a fuel assembly from outside the pressure vessel. Openings dispersed throughout the fuel assemblies allow for the insertion of the control rods. Each fuel assembly includes such openings for purposes of interchangeability even though a control rod is not used with each fuel assembly within the core. A control rod drive mechanism is used with each control rod to move the control rod into and out of the nuclear core. The drive mechanisms are each attached to the top of the pressure vessel. Electrical power is used to operate the control rod drive mechanisms.
Each of the elongated rods making up the control rod contain or comprise materials which absorb neutrons produced by the fission process of the nuclear reactor. Materials having a high neutron capture cross section are most often used for control rods. Accordingly, such materials as boron carbide, hafnium, or a combination of silver-indium-cadmium have been used successfully.
The prior art control rod drive system used in present day pressurized water nuclear reactors has not changed in any substantial manner since its initial application. The control rod drive mechanisms each consist of a stationary gripper and coil to hold the rod when it is not in motion. A movable gripper and coil holds the rod when the rod is being moved in stepped increments. A lift coil moves the movable gripper and rod a discrete distance or step. The three mechanisms are sequentially activated to produce a step up (the movable gripper is engaged before the lift coil is energized) or to produce a step down (the movable gripper is engaged after the lift coil is energized). The coils associated with four control rods (comprising a group) are jointly activated in the appropriate sequence by electrical apparatus housed within a single control cabinet so that the group of four control rods move up or down together. Each cabinet contains three controlled rectifier bridges, with one bridge controlling the current to the stationary gripper coils of four rods, another bridge controlling the movable gripper coils of the four rods, and the last bridge controlling the lift coils of the four rods. A second cabinet to control the coils of four other rods is used in conjunction with the first cabinet to move a second set of four rods in conjunction with the first set of four rods. The total of eight rods (two groups of rods) and eight separate drive mechanisms, comprise a "bank" of rods, with the rods being located symmetrically around the core to produce fairly uniform radial neutron flux perturbations. Each of the two groups of control rods comprising one bank are moved alternately to produce bank motion and to assure and maintain the previously noted symmetrical radial neutron flux perturbation. There are a number of banks of control rods in each nuclear reactor. Each bank of eight control rods has approximately the same reactivity worth. Sequential banks of control rods are maintained in a fixed separation or overlap which is kept uniform between all banks to produce a substantially uniform rate of reactivity insertion or removal.
Development of the control system for the control rod drive mechanism led to the use of one control cabinet for three groups of control rods. Within the cabinet, one bridge is used for the movable gripper coils and one bridge is used for the lift coils of all three groups of control rods. This is possible because at any one time only one group is moving while all other groups are stationary. On the other hand, separate bridges are required for the stationary gripper coils of each group of control rods. Thus, separate bridges are made available for the stationary gripper coils of each group of rods but the other bridges within the cabinet may be shared among the groups of rods by the use of appropriate switching circuitry. Accordingly, each control cabinet in the prior art contains five bridges plus power switching circuitry. Further attempts to include the control of the movement of additional groups of rods within a single cabinet were deemed futile because it required a cabinet of too large a physical size. It would be advantageous, obviously, to further extend the concept of sharing of electrical current control equipment to more than three groups of rods while maintaining the standard size control cabinet. Further, sharing of power control apparatus is, accordingly, an object of the present invention.
The design function of the control rod systems of the prior art, including the one described above, is to respond to mismatches or differences in the system temperature or turbine power from the reference values or demand values. In other words, to control the overall power output from the power plant. Although the insertion of the control rods in the nuclear core produces undesirable neutron flux perturbations, especially in the immediate vicinity of the control rods, which adversely affects the maximum operating power level of the nuclear reactor and, hence, the power plant, few achievements have been made whereby the operation of the control rods is used to shape the neutron flux throughout the nuclear core and to control the operating power level.
Flux perturbations are produced by other phenomena in addition to the control rods. The buildup of xenon, which is a nuclear poison, within the fuel elements during reactor operation also affects flux distribution. The abrupt ending of fuel elements at the radial periphery of the core also affects flux distribution. Similarly, the upper and lower axial limits of the fuel assemblies affects flux distribution. Proper flux distribution is very beneficial to reactor operation, power output, fuel life, fuel economy, reactor safety, and costs, just to mention a few.
In the past, part length control rods were utilized to damp any xenon-induced oscillation affecting the flux distribution. The subsequent determination that part length rods could potentially adversely affect the shape of the flux led to their prohibition altogether. This then left the entire task of flux shaping and power level control to the full length control rods. Later, the concept of a "light" bank of control rods to primarily control the power output, while the other banks remained at normal worths, was introduced. This concept represented a significant improvement in reactor capabilities without requiring any significant change in the control rod system. Shortly afterward, the "light" bank concept was extended further to "ultra-light" banks and "gray" banks control concepts. These, however, were rejected for actual use because their use would have required major changes in the control rod assemblies and/or the rod control system design.
Accordingly, new and improved methods of reactor power control in conjunction with neutron flux shaping are required and are objects of the present invention.
Another object of the present invention is to provide such power control and flux shaping without requiring major design and hardware changes to the presently known nuclear reactors.
Another object is to provide such power control and flux shaping on a back-fittable basis to presently existing plants.